The present invention relates to a secondary air control system in an internal combustion engine.
The system comprises means for removing harmful emissions in the exhaust gas and means for supplying secondary air to said removing means. In order to decrease CO (carbon monoxide), HC (hydrocarbon) and NO.sub.x (nitrogen oxide) in the exhaust gas, a three-way catalyzer is arranged on the exhaust pipe of the engine. The three-way catalyzer oxidizes CO and HC and simultaneously dioxidizes NO.sub.x, thereby purifying the exhaust gas. The three-way catalyzer effectively acts simultaneously upon the three harmful components only when the exhaust gas is stoichiometric. Accordingly, the air/fuel ratio must be precisely controlled in order to attain the stoichiometric exhaust gas. For this purpose, one method has been proposed wherein the secondary air is injected into the exhaust pipe upstream of the three-way catalyzer. The injection quantity is controlled so that the gas in the exhaust pipe has stoichiometric components. In such a method, an oxygen sensor is generally used for detecting the oxygen ratio in the exhaust gas in order to control the secondary air quantity in response to the output signal of the sensor. However, in such a control method, the secondary air quantity cannot be precisely controlled because the sensing ability of the oxygen sensor is lowered after a long use. Another secondary air control method has been proposed wherein an oxygen sensor is not used. In this second known method, the secondary air is supplied in response to the intake air quantity of the engine. The intake air quantity is calculated on the basis of the intake vacuum of the engine and the number of revolutions per minute of the engine. However, in this second method, the calculation reliability is low because the two signals, intake vacuum signal and engine revolution count signal, are required for the calculation, both of which signals frequently vary in response to the driving condition of the engine.